1. Field of the Invention
The present invention relates to RF bias isolation circuits, and, more particularly, to RF bias isolation circuits monolithically realizable on a semiconductor substrate.
2. Description of the Related Art
In the design of electronic circuits that operate at radio frequencies (RF) and above, it is necessary to provide a means for selective electrical isolation of the DC power supply node from the RF signals present in the circuit. Such means should allow DC currents to flow with little or no restriction while at the same time impeding the flow of RF currents. The traditional solution to this problem is known as a "bias tee" and is shown schematically in FIG. 1. In this approach, the DC supply voltage V.sub.DC is applied to node 10 and DC current I.sub.DC flows through inductor 20 to node 12, where it is used to bias the circuit (an active circuit is shown by way of example, but the bias could also be applied to a passive circuit, such as a PIN diode switch). The current I.sub.DC is prevented from flowing to node 14 by blocking capacitor 22 and from flowing to ground by bypass capacitor 24. To the RF current I.sub.RF, the inductor 20 is a high impedance choke and the blocking capacitor 22 is a short circuit and therefore I.sub.RF flows freely to node 14 but is prevented from flowing to node 10.
The inductor 20 and bypass capacitor 24 form a series-resonance circuit with a resonance frequency defined by: ##EQU1## In effect, this is a low-pass filter with its band-stop at the desired RF frequency. To implement such a circuit with f.sub.o in the UHF band (for example, 815-925 MHz, as required in cellular telephone communications) would require: EQU L.sub.RFC &gt;30 nH EQU C.sub.BP &gt;1000 pF
The high value of L.sub.RFC required would make it impossible to implement the circuit of FIG. 1 in a low-cost, manufacturable monolithic integrated circuit. Inductance values greater than 10 nH are very difficult to obtain due to the large area required by the inductor, its current handling capability and excessive losses. For such inductors, traditionally, wound coils or printed circuit spirals coated with ferrite materials have been used. Both such solutions are obviously incompatible with integrated circuit technology.
Another alternative, illustrated schematically as FIG. 2, is to use a smaller value of inductance L.sub.RFC and incorporate inductor 20 as part of the RF output matching circuit, since the smaller value of L.sub.RFC will no longer be "invisible" to the RF signal.
The problem with this approach is that the RF operating frequency band of the host circuit is closer to the resonant frequency of the inductor 20/capacitor 24 choke. At these frequencies, the choke is no longer seen as a high impedance by the RF current, but instead appears inductive. The Q-factor of this shunt inductor 20/capacitor 24 choke is: ##EQU2## where: R.sub.s =series resistance of inductor 20 and capacitor 24
w=RF frequency in radians
Therefore, decreasing the value of L.sub.RFC lowers the Q-factor of the shunt inductor 20/capacitor 24 choke. Because the Q of the external bypass capacitor 24 is normally low, substantial loss in the RF signal occurs due to the relatively large RF current that flows in the inductor 20/capacitor 24 path.
FIG. 3 shows a computer analysis (using EEsof Touchstone analysis software) of the RF signal loss of the circuit in FIG. 2 assuming the following values (which correspond to a RF band of 800-1000 MHz):
C.sub.BP =10660 pF PA0 R.sub.s =1 Ohm PA0 L.sub.M =5.5 nH PA0 C.sub.M =5.0 pF PA0 C.sub.B =24 pF PA0 L.sub.RFC =varied parametrically as shown in FIG. 3.
It is readily apparent that the RF signal loss is directly proportional to the inductance L.sub.RFC. A loss of less than 0.1 dB (required in a high efficiency amplifier, for example) requires an inductance L.sub.RFC of greater than 9 nH. Therefore the circuit configuration of FIG. 2 requires inductance values which are not compatible with monolithic integration. It also makes the RF signal loss very sensitive to the Q-factor of the external bypass capacitor 24 which is not desirable.
Therefore, the need exists for a RF bias choke that is implementable in a monolithic form and which will cause minimum loss to the RF signal (i.e. high efficiency).